This invention relates to antennas, reflectarray antennas, and specifically to an electronically scanned reflectarray antenna.
A reflectarray antenna is described in a paper by D. G. Berry et al., xe2x80x9cThe Reflectarray Antennaxe2x80x9d, IEEE Transactions on Antennas and Propagation, November, 1963, pp. 645-651. In a basic reflectarray 100 shown in FIG. 1, the amplitude and phase of electromagnetic fields reflected from a surface 105 at any point are determined by the surface impedance at that point. A scanned radiation pattern can be generated by variation of the surface impedance along the two-dimensional (x, y) reflecting planar surface 105. The reflecting surface 105 is spatially excited by a wave 125 from a feed horn antenna 120 or other suitable radiating element, as shown in FIG. 1. The radiated beam 110 from the reflecting surface 105 is a collimated plane wave that may be one of the following forms: a pencil beam parallel to a bore site axis of the array 100, a broad beam or fan beam in one or two dimensions parallel to the bore site axis of the array 100, or a pencil beam pointing at any angle xcex8 and xcfx86 from the surface 105 less than end-fire relative to the axis normal to the reflecting surface 105.
The tailoring of the surface impedance to obtain a desired radiation pattern may be accomplished in one of two ways. The first method is a continuous surface impedance variation obtained by varying the thickness, d, of a grounded dielectric slab 115 in FIG. 1. The second method is an approximation of a non-uniform surface impedance reflecting surface 105 utilizing radiating elements terminated in shorted transmission lines of various lengths distributed across an array. The second method may be accomplished by dipole radiating elements terminated in shorted transmission lines or open ended waveguides 320 terminated in short circuits as shown for an array 300 in FIG. 6. Other radiating elements, such as microstrip patches, are also possible. In both of these methods the surface impedance is variable along the reflecting surface 105 but is fixed at any one point.
The basic reflectarray architecture has been improved within the existing art to obtain electronic beam scanning by incorporating traditional phase shifter technology, such as PIN diode phase shifters. PIN diode phase shifters have been used in a shorted-circuit microstrip transmission line terminating the radiating elements. The referenced paper discloses a waveguide reflectarray that has switching diodes placed at appropriate intervals from a waveguide aperture to change the distance to a short circuit termination to rapidly electronically scan the antenna. The PIN diode and varactor based reflection phase shifter prior art has several disadvantages. High quantization side lobe levels are present due to a finite bit count of digital phase shifters. Large component counts for electrically large phased arrays are necessary. A 4-bit switched line phased shifter requires 16 diodes per phase shifter. Complex electrical interconnect, both in terms of RF lines, DC bias, and digital control lines are required. Complicated assembly techniques are required with the high component count and complex electrical interconnect. The PIN diode and varactor based phase shifters have maximum RF power limitations.
Waveguide and horn reflectarrays have been implemented using mechanically movable waveguide shorts as disclosed in U.S. Pat. No. 6,429,823. Large arrays at high frequencies make this approach difficult to implement due to the small size of components and the mechanical complexity of the large array due to the number of waveguide elements and the use of motors to move the shorts. An additional disadvantage of motorized moveable waveguide shorts is the slow movement of the shorts and the resulting slow scanning of the antenna beam.
What is needed is a reflectarray antenna that has the capability to scan a radiated beam with more efficient and cost effective phase shift functions.
A reflectarray antenna having a scanned radiated beam is disclosed. The reflectarray antenna comprises an antenna feed for radiating a wave. A reflecting surface located on a tunable substrate is excited by the wave from the antenna feed. The reflecting surface reflects the wave in accordance with a variable surface impedance of the tunable substrate modulated to scan the radiated beam.
The tunable substrate may be a dielectric slab of ferroelectric material having a dielectric constant modulated by varying a DC electric field to scan the radiated beam. The reflecting surface may comprises traces in selective locations on the reflecting surface to provide the DC electric field to vary the surface impedance to scan the radiated beam. The tunable substrate may be a dielectric slab of ferromagnetic material having a permeability modulated by varying a DC magnetic field to scan the radiated beam.
The tunable substrate may be an electromagnetic band gap (EBG) material electromagnetic crystal (EXMT) structure to vary the surface impedance of the reflecting surface to scan the radiated beam. The EMXT structure is a dielectric substrate of ferroelectric EBG material having a dielectric constant modulated by varying a DC electric field to scan the radiated beam. The EMXT structure may also be a dielectric substrate of ferromagnetic EBG material having a permeability modulated by varying a DC magnetic field to scan the radiated beam. The EMXT structure may also be a dielectric substrate of semiconductor EBG material and a plurality of diodes on the dielectric substrate reverse biased to act as variable capacitors to modulate the surface impedance of the reflecting surface to scan the radiated beam. The EMXT structure may be fabricated on a semiconductor wafer.
The reflectarray antenna may also comprise an antenna feed and a reflecting surface located on a plurality of short-circuited waveguides for reflecting the wave from the antenna feed in accordance with a variable surface impedance modulated to scan the radiated beam by adjusting phase shifts of the waveguides with substrate-based phase shifter located in the waveguides.
Each of the short-circuited waveguides may comprise a bulk dielectric ferroelectric-based phase shifter located on an end cap of the waveguide. A bias electrode connected to a bias feed applies a bias to vary the phase shift by varying the dielectric constant of the bulk dielectric ferroelectric phase shifter. Impedance transformers are used for matching the ferroelectric phase shifter portion of the waveguide to an air filled portion of the waveguide.
Each of the short-circuited waveguides may comprise a tunable electromagnetic band gap material EMXT structure for phase shifting located on two walls of the waveguide. The tunable electromagnetic band gap material varies the phase through an adjustable DC bias on the electromagnetic band gap material. A short circuit at the end of the waveguide for reflects the wave. The EMXT structure may be a dielectric substrate of ferroelectric EBG material having a dielectric constant modulated by varying a DC electric field to scan the radiated beam. The EMXT structure may be a dielectric substrate of ferromagnetic EBG material having a permeability modulated by varying a DC magnetic field to scan the radiated beam. The EMXT structure may be a dielectric substrate of semiconductor EBG material and a plurality of diodes on the dielectric substrate reverse biased to act as variable capacitors to modulate the surface impedance of the reflecting surface to scan the radiated beam.
The reflectarray antenna short-circuited waveguides may be a tunable electromagnetic band gap material EMXT structure for phase shifting located at an end of the waveguide to create a reactive impedance termination for reflecting the wave.
The reflectarray antenna with a scanned radiated beam may comprise an antenna feed for radiating a wave and a reflecting surface located on a plurality of short-circuited waveguides for reflecting the wave from the antenna feed in accordance with a variable surface impedance modulated to scan the radiated beam by adjusting phase shifts of the waveguides. Each of the short-circuited waveguides comprises a metallized piezoelectric shorting surface located in the waveguide to vary the length of the waveguide when a bias is applied.
It is an object of the present invention to provide a reflectarray antenna that has the capability to scan a radiated beam in two dimensions using efficient and cost effective phase shift functions.
It is an object of the present invention to realize an electronically scanned reflectarray antenna that incorporates tunable substrate electromagnetic band gap materials.
It is an object of the present invention to realize an electronically scanned reflectarray antenna that incorporates shorted waveguides with phase shifters and waveguides with moveable shorts.
It is an advantage of the present invention to reduce the complexity of phase shift control circuits.
It is an advantage of the present invention to provide a continuously variable phase shift across a reflecting surface in a reflectarray antenna.
It is a feature of the present invention to reduce feed complexity by replacing a constrained feed with a spatial feed.
It is a feature of the present invention to provide conformal reflecting surfaces.
It is a feature of the present invention to provide twice the phase shift with a shorted waveguide approach.